Location systems attempt to provide estimates of the velocity state of an object (velocity state estimates). An object whose velocity state is not known α priori, and whose velocity state is to be estimated by the location system, is referred to as a target object; an object whose velocity state is known, and whose velocity state is used as a reference for the estimation of other velocity states, is referred to as a reference object. The velocity state is the set of all characteristics of an object relating to its status in space and time, including such characteristics as the position, velocity, acceleration, orientation, rate of rotation, axis of rotation, direction of rotation, and any other parameters that apply to its status in space and time.
Many location systems use a “velocity model” to estimate locations of objects in the absence of direct measurement of location—for instance between measurements, or when objects are in areas not covered by the system, such as in buildings or tunnels. A velocity model is an active representation of a velocity state that may provide velocity state estimates for an object even for times when velocity state measurements (direct measurements of data relevant to the construction of a velocity state relative to reference objects with known velocity states) are unavailable. A velocity model may include estimates of some or all of the velocity state parameters, and may also include additional data such as error estimates, initialization history or change history, and identification of reference objects used in velocity state measurements. In many cases, the velocity model is simply comprised of the position, velocity, and time of the most recent velocity state measurement (or from several velocity state measurements) and the current position state of the object is estimated from the initial position and velocity based on the assumption that the object with which the velocity model is associated undergoes no acceleration during the interval between location measurements.
It is important to note that there is not a direct correspondence between parameters of a velocity state, parameters of a velocity model, parameters of a velocity state estimate, and parameters of a velocity state measurement. For example, a velocity model or velocity estimate may contain parameters representing estimated uncertainties, whereas a velocity state by definition has no uncertainty, and a velocity state measurement may have parameters which do not enable the estimation of any particular parameter of a velocity state, but which must be combined with other velocity state measurements in order to determine estimates for such parameters as position or velocity. Most velocity models, velocity state estimates, and velocity state measurements do not contain all of the parameters of a velocity state, and most contain other parameters relating to the specific implementation of the location technology.
Inertial navigation systems (“INS”) provide a greatly improved method of maintaining a velocity model, by measuring acceleration on one or more axes, both linear and rotational. These acceleration measurements are used to update the estimate of parameters such as velocity and orientation in the object's velocity model. INS have long been used in missile and spacecraft guidance systems, and with the advent of micro-machining INS are finding their way into the personal (portable) devices market—devices which could be worn by people or used on small assets. Some such systems are purely inertial, measuring rotational acceleration by the force required to change the rotational axis of a gyroscope; other (usually lower cost) devices augment the purely inertial measurements with magnetic compasses, altimeters, or other devices. Devices using a six-axis gyroscope method are generally the most expensive and the most accurate.
If a target object has a velocity model (such as that maintained by an INS device) for location estimates, a reference velocity state estimate must be obtained through means other than the INS, since an INS system measures acceleration and estimates parameters such as position and velocity only as differences from an initial state. A velocity model may contain a reference velocity state estimate and an estimate of the difference between the reference velocity state estimate and the current velocity state, deriving a velocity state estimate by adding the difference to the reference velocity state estimate; alternatively, it may contain a velocity state estimate which is initialized to a reference velocity state estimate and continually maintained to represent the current velocity state estimate. Target objects that include an apparatus to assist in the determination of their velocity state estimate (target devices) may use appropriate location systems, for example the Global Positioning System (“GPS”) to obtain velocity state measurements from which to derive an initial velocity state estimate. Alternatively, they may assume aspects of an initial velocity state estimate by, for example, requiring that the object remain motionless until initialization is complete and assuming an initial velocity of zero. An initial velocity state estimate may be determined from a single velocity state measurement, or it may be integrated from several velocity state measurements. Other location methods known in the art may be used to obtain an initial velocity state estimate, including simply requiring that the velocity model be initialized near a predetermined location. The initial velocity state measurement thus provides data for the initial velocity state estimate of the velocity model. The difference between the initial velocity state estimate and the current velocity state estimate is zero at the time the initial velocity state estimate is obtained. The parameters in a velocity model or velocity state measurement are generally associated with a reference frame; for instance, the GPS system uses the surface of the Earth as its reference frame, and uses the satellites as reference objects. The velocity model may be stored and maintained on a target device, or a target object may have a velocity model that is stored and maintained on a remote device. The velocity model parameters may be represented in any number of forms. For instance, values may be represented in rectilinear or polar coordinates; velocity may be represented by a vector sum or a speed and direction; linear or rotational speed may be represented as momentum or kinetic energy, with explicit or implicit values of mass or moment; and other variations too numerous to mention are available. Systems may also use techniques such as averaging or predictive filters to obtain an estimated velocity state from multiple location estimates.
The velocity model may be compromised by any number of factors. For instance, transient errors in acceleration measurement result in constant errors in the velocity estimate, which in turn result in constantly increasing errors in the position estimate. Under the best conditions, normal calibration and measurement errors will gradually accumulate over time, eventually resulting in large errors in the velocity model. Environmental stress, shock, and other conditions may introduce additional errors into the measurements, in many cases causing the velocity model error to quickly degrade to the point of uselessness.
A number of approaches to mitigating the compound error properties of the INS velocity model have been implemented. Many location devices, such as INS, that use velocity models rely under normal circumstances on velocity state measurements, reverting to velocity state estimates based on velocity models only at times when velocity state measurements are unavailable. Others use techniques such as performing periodic adjustments of their velocity models, for example by requiring a user to stop for a certain amount of time in order to reestablish their calibration to zero velocity. However, recalibration of the velocity parameter does not correct any errors in the position parameter that were accumulated while the velocity estimate was in error.
Many devices are able to track estimated error over time. Tracking estimated error over time does not improve the accuracy, but only warns a user about the potential for error in the estimate offered so that the user can behave accordingly. The error may be represented, for example, in the form of a probability distribution function or in the form of a radius representing a maximum expected error at some pre-established level of certainty.
One approach to maintaining a velocity model for a target object is to supplement it with a service providing periodic velocity state measurements. GPS is one such service, but GPS will not penetrate most buildings, and even if it does, it will not deliver the location accuracy required for useful in-building location. A local RF solution would be better for indoor conditions; such systems include those which, like GPS, derive location from the difference between the propagation delay between a target device and a first reference object and the propagation delay between the target device and a second reference object (this technique is known as time difference of arrival (“TDOA”)), and those systems which derive location from the round-trip propagation delay between the target device and a single reference object (this technique is known as ranging). However, RF location systems with sufficient bandwidth to provide accurate location generally do not have sufficient power to guarantee coverage throughout the building. Using such systems improves the situation, but leaves a number of cases where devices may remain out of coverage for longer than their velocity model can be considered accurate; for instance, if asset tracking devices are stored outside system coverage, or if firefighters or law enforcement officers are involved in an extended operation outside system coverage or within buildings where coverage is unavailable.
Thus, there exists a need for a method to improve or maintain the accuracy of the velocity model of a target object without the need to bring the target object to a reference object in a known or fixed location, and which does not depend on the ability to establish RF or other links with a reference object in a known or fixed location.